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MojoTone 5e3 Kit woes

Discussion in 'Shock Brother's DIY Amps' started by 4075aaron, Feb 15, 2021.

  1. no doz

    no doz Tele-Meister

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    ^ this is very good advice!

    here's a copy of the voltage chart i used when testing the DC voltage in my 5E3, it might help guide you when you're taking your measurements. your goal voltages will obviously be a bit different than the specific ones listed here, but it will show you where to measure and it has nice illustrations of what each pin terminal is doing in relation to the tube.

    [​IMG]

    as always, make sure to observe proper safety precautions. dangerous stuff
     
  2. Nickfl

    Nickfl Friend of Leo's

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    Yes, that's what I meant. so that tells us the problem is not in the power app it's somewhere in the face inverter or preamp. Now do the same test on each of the preamp tubes, working back towards the input. Probe the plate and grid pins and see if you get some sound. It won't necessarily be very loud but you should hear something at each of these locations and if you don't that's probably where you should start looking for the problem.

    Also go ahead and make the full voltage chart others have been asking for while you're doing this and take a look at the shorting tabs on your input jacks as someone else suggested. Those are the tabs that make a connection between the tip and the center solder lug when nothing is in the jack and break the connection when a cable is inserted. Sometimes they may be bent a little bit and not open up all the way when a plug is put in they usually if they have a problem it's the other way around and they don't short with a plug is taken out which can cause the amp to be noisier than it should at idle.
     
    Last edited: Feb 17, 2021
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  3. D'tar

    D'tar Friend of Leo's

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    [​IMG]
     
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  4. King Fan

    King Fan Poster Extraordinaire Ad Free Member

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    @4075aaron , I like the way you're responding and staying up with all this advice. Yeah, a voltage chart like the one above is helpful. That's a nice example. If it helps, you may be able print this one out and fill it in, take a phone pic, and post inline here. But many folks just do grid layout on paper, and some just type it all in.

    upload_2021-2-17_8-14-1.png

    It remains to be seen if your V2p7 is actually a problem -- I included the important note by @D'tar above.

    I'm glad your speaker works. As @Nickfl says, if your amp passes the pop test at least at the 6V6 plates, the speaker output is at least partially working. To check it's 100%, the wiring of the output jack and on to the speaker can easily be tested by skipping over it. Clip alligator leads to the lugs with the OT secondaries on the speaker jack, clip their other ends to your speaker lugs.

    Do work the pop test across the sockets as he says; in a working amp the pops get louder -- more amplified -- as you work back to V1.

    Back on the subject of jacks, what the guys are asking about your speaker and input jacks is if the shunt prong separates nicely when you insert a plug. The shunt prong is the one that leans on the tip prong when a plug is removed -- and they're saying you should have shunt prongs on the inputs and speaker jack, but no such prong on the aux jack.

    A good way to triple check your input wiring (we're almost past the board and tubes, aren't we? have to look everywhere) is the D'tar test . Aha, I see he just posted it!!! Great minds work alike... and mine tags along.
     
    Last edited: Feb 17, 2021
  5. goldguitarguy

    goldguitarguy Tele-Meister

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    This is the shunt prong. When you insert your cable jack it breaks this connection.
     

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  6. 4075aaron

    4075aaron TDPRI Member

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    So it is late, and I see that I have a lot of replies that I need to review. Work and personal life did not let me spend as much time on the amp over the past three days as I was hoping. But I wanted to get the voltage readings to all of you for your input. I will list these starting from what MojoTone (and other manufactures) call V1 to V4. V1 is the first preamp tube on the far right when looking into the chassis (the 12ay7) and V3 and V4 are the two 6v6s power tubes (there is not much to report on them). I will provide V5 readings (rectifier tube) tomorrow...it is late tonight and I will want to unplug all of the tubes to to also provide the secondary readings (although I have never had any issues with the secondary readings with this amp in the past 7 months of building).

    I will list the tube - tube value - P number - (what the reading should be) what my reading is for each. "HW" means heater wire, which I am reading 6.4v at all points. If there is a "?" that means the wire goes into a pot and I am not sure what reading I should or I have:

    V1 (12ay7):

    P1 – (125v) 122.3v
    P2 – (?) 1.4m
    P3 – (2.1, to P8) 2.1v
    P4 – (HW)
    P5 – (HW)
    P6 – (125) 121.9v
    P7 – (?)
    P8 – (2.1, to P3) 2.1v
    P9 – (HW)



    V2 (12ax7):
    P1 – (160v) 157.4v
    P2 – (?) ?
    P3 – (1.4v) 1.24v
    P4 – (HW)
    P5 – (HW)
    P6 – (198v) 193.8v
    P7 – (19v) 3.14v (46v at the resistor junction)
    P8 – (53v) 49.4v
    P9 – (HW)

    V2 P7 is the outlier and what is causing me issues. The tube glows, but weakly. I measured the voltage at the resistor junction of the 56K, 1M, and 1.5K resistors as suggested earlier and found the voltage to be 46v.

    Lastly, I am having a hard time getting HD pics of my tube sockets, but will try more tomorrow. I am also in the process of replacing the Silver Mica Cap across the pots; I don't believe that it is bad, but before I started this thread I was in the process of replacing every single element in this amp because I have no idea why it is not working.

    Here are some pics; I can try and get better ones tomorrow (the input wiring might be of most interest to you all).

    Thanks in advance, and I will try to reply to all of your posts over the past three days tomorrow!

    Sorry...pic one is the input wiring, the remaining three are tube wirings (I can't seem to edit them, but V2, 3, and 4).
     

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  7. 4075aaron

    4075aaron TDPRI Member

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    This was a great suggestion. My speaker input was fine with this, but none of my instrument inputs were connecting like this. Unfortunately after bending the prongs so that they would break the connections with an input, I still get no sound. But without your input, I still would have been lost once figuring out the V2 P7 voltage issue. Thanks!!!
     
  8. 4075aaron

    4075aaron TDPRI Member

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    Update!!! goldguitarguy to the rescue! I took a second look at what you posted, and a second look at my jack inputs, and you were right. None of them were connecting/disconnecting correctly. I updated based on our post, and the amp came to life!!!

    It sounds great!!! No ground hum...everything is like it should with a 5e3. None of the prior amps that I have built in the past needed this sort of attention to the input jacks. But I am glad that goldguitarguy pointed this out to me.

    Thanks to everyone who provided input! Big thanks to those who let me know that 19V is not really needed at P7 on V2. The amp sounds fantastic.
     
  9. goldguitarguy

    goldguitarguy Tele-Meister

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    I can’t take all the credit for that. A few people mentioned it I just posted the picture...lol... Fantastic you got your amp working!
     
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  10. no doz

    no doz Tele-Meister

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    glad it's working! congrats on finally solving the mystery, have fun
     
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  11. dan40

    dan40 Friend of Leo's

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    Congratulations on finding the problem! I have has issues with both Switchcraft and Cliff jacks in a couple of new builds recently. Not sure if quality control is slipping or if I just got a couple of bad jacks.
     
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  12. 4075aaron

    4075aaron TDPRI Member

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    Again, thanks to everyone for all of their advice. I did not get much time today to play with the amp. I still have a little work to do on it (there is a ground hum on the Bright channel but I think that I know why (again, the input jacks)). A quick reminder if possible, I seem to remember back when I was building amps that it is good practice to turn it on and leave it running for several hours after having a solid build. Is that still considered good practice?
     
  13. dan40

    dan40 Friend of Leo's

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    "Burn in" is the term that you are referring to. Many builders will let the amp sit powered up for several hours after a new build to make sure that no surprises occur. Many folks also say that the amp will settle in after several hours of use though I can't say that I have ever experienced much of a change in the amp's sonics in the first few days of use. As for the hum in the bright channel, be sure all of the solder joints from the input jack to the bright channel's volume pot are good. Pay special attention to the grounding solder joints. Also be sure the open chassis is shielded with a metal cover if it's sitting out on your work bench. Once installed in the cab, the back panel usually has some sort of shielding attached to help the circuit stay quiet. When the chassis is open on your work bench many sources of hum including light fixtures around your bench can cause noise in the circuit.
     
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  14. 4075aaron

    4075aaron TDPRI Member

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    Yeah...the shielding on the back cover is something that I will need to add. This Mojotone kit did not have any shielding on the back panel which I thought was odd. I think that they tried to make the kit super vintage or as close to an original 5e3 as possible, with the option of the old AC wiring (with "death cap") and no shielding. This weekend I will pick some aluminum tape and see if that makes a difference. Even with the wood panel on, the hum has almost completely gone away. As it is now, it is a great sounding 5e3 and all of the knobs interact the way they should!
     
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  15. pfarrell

    pfarrell TDPRI Member

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    After completing my 5E3 build—no real drama—the other day after about an hour of messing around playing/practice—I suddenly started getting some static, fuzz, volume loss, never a complete sound cutout—but weird. Messing with the NFB switch seemed to be related (LOVE this switch BTW)—I ruled out the cables I made—seemed like it could have been a lot of potential issues—solder joints? (I had already diagnosed and fixed a missing solder connection on a heater wire! That took a while to figure out—it was perfectly hidden in that vintage style fat green wire)...

    After taking a chopstick poke around—while strumming, it finally occurred to me to check those jack switches—and a wiggle of the output plug told the story...the output one was iffy! Little bending—and all was well—it was A LOT of drama for such a simple issue—why it happened after days and days of use and a warmup??????

    Question—and real reason for this post—is there a BETTER way? Better switch? Use the Neutrik plastic ones and ground them? Had a similar issue on my Champ build with an input. I mean—just getting them shipped to you it seems that they can get bendy...
     
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  16. King Fan

    King Fan Poster Extraordinaire Ad Free Member

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    Good question. Yesterday I was messing with a buzzy speaker and decided to use my ol' reliable 5F2a, my first build, as a test bed. It has always worked perfectly. But when plugging in the suspect speaker, I noticed the speaker jack shunt prong didn't separate much (didn't look separated). Hmm, same thing with the old plug. The amp worked and had always worked, but the shunt must have been open by only the tiniest margin.

    Uglier story. My buddy ordered a big Fender clone from a boutique shop. It got here and almost immediately was cutting in and out. He got into a nasty email discussion with the shop; he wanted it replaced, they wanted him to pay shipping both ways, since it had worked fine when they shipped it. I dropped the chassis and plugged in -- yep, the shunt prong was 99% leaning on the tip prong. A few seconds with a screwdriver and the amp has worked perfectly ever since.

    I love Switchcraft jacks and use them 100%, but their design sorta means the shorting version are prone to this problem. The good news is the problem is easily tested and easily fixed. Insert a plug (a good standard name brand plug -- some ¼" plugs are actually slightly undersize.) You should be able to easily see daylight between tip and shunt.

    IMG_6588.jpg

    Failure to separate well is most common fault -- fix by gentle prying force between shunt and tip with bladed screwdriver. You want them both parallel to the plug when open. Re-test. Increase force only bit by bit -- opening too far is a bit harder to fix, but usually not impossible.

    Failure to seat firmly is less common, but a lot more subtle -- the jack will work when a plug is inserted, but it won't protect you from noise (input) or OT damage (output). You can inspect visually, but a no-plug continuity test from G to Sw is worthwhile. Seen from the lug side, the topology is like this:

    upload_2021-3-1_8-57-22.png
     
    Last edited: Mar 1, 2021
  17. Phrygian77

    Phrygian77 Friend of Leo's Gold Supporter

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    It might be obvious to some folks, but I just wanted to point out, if you're doing this, pry away from the contact points. You don't want to damage the nickel plating at the contact.
     
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  18. King Fan

    King Fan Poster Extraordinaire Ad Free Member

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    Good point, yes, I insert the screwdriver at the base of the prongs -- you want to realign them, not curve them.
     
  19. bluesintheblood

    bluesintheblood TDPRI Member

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    Man, I'd really like one of their amps to build but just seeing so many threads recently with issues, how many of these issues have to do with a lack on mojotone's part of including any INSTRUCTIONS??

    How hard is it to draft up some instructions??
     
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  20. King Fan

    King Fan Poster Extraordinaire Ad Free Member

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    Interesting question. Actually, it is hard, and we can say that for two reasons a) if you've built one, think of how long and complex instructions would be, and how hard it'd be to make 'em clear b) folks have tried it; in fact StewMac sells Mojo kits with instructions and the steps are long, complex, and occasionally, um, unclear or suboptimal. Heck, TubeDepot sells a 5F1 kit with a PCB, and supply detailed instructions *and* a video. Both are highly complex, even for that little amp, and folks still often find they've made 'mistakes' in the build.

    I built a Mojo PR without instructions, and it *was* hard, and I learned a lot, and the big problem I had on startup probably would not have been prevented by instructions -- I knew what to do, but just got careless late one night doing it. :)
     
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